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Paquet, M, Day JMD, Castillo PR.  2019.  Osmium isotope evidence for a heterogeneous 3He/4He mantle plume beneath the Juan Fernandez Islands. Geochimica et Cosmochimica Acta. 261:1-19.   10.1016/j.gca.2019.06.039   Abstract

Mantle plume models have been widely applied to explain the formation of ocean island basalts (OIB), with high-3He/4He in their lavas being explained by sampling of a primitive deep mantle source. The Juan Fernandez Islands have 3He/4He (7.8–18 RA) similar to or higher than in mid-ocean ridge basalts (MORB; 8 ± 1 RA) and have been used to both support and refute the mantle plume hypothesis. Ambiguity regarding the origin of the Juan Fernandez Islands primarily originates from interpretation of mantle source signatures between the lava series from the two main islands, Robinson Crusoe and Alexander Selkirk. To examine this issue, we report new whole-rock and olivine separate 187Os/188Os ratios and major-, trace-, and highly siderophile-element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundances. The HSE and trace element abundances in Juan Fernandez main shield lavas can be explained by up to 30% olivine removal, together with spinel crystallization at 1–5 kbar, whereas Robinson Crusoe rejuvenated lavas can be reproduced by higher-pressure fractional crystallization (up to 10 kbar). An assemblage of 30 modal % olivine and 1–5 modal % spinel, combined with the additional contribution of primary melt trapped in olivine inclusions reproduces the range of HSE compositions observed in Juan Fernandez Archipelago olivine grains. Ratios of 187Os/188Os for Juan Fernandez lavas are generally less radiogenic than global OIB and show no correlation with indices of fractionation, indicating that they reflect mantle source compositions. Younger basanite lavas from Robinson Crusoe represent rejuvenated volcanism dominantly from a depleted lithospheric mantle source (187Os/188Os < 0.13) mixed with a high-3He/4He component from the main shield building stage. These lavas are similar in origin and composition to other Pacific rejuvenated lavas (e.g., Samoa, Hawaii). Robinson Crusoe main shield lavas are from a high-3He/4He (>18 RA) and enriched mantle source (187Os/188Os = 0.1312) similar to the ‘C’ or ‘FOZO’ component, whereas Alexander Selkirk lavas are consistent with a dominant contribution from a depleted, low-3He/4He (<10 RA) mantle component. The mantle sources of the shield lavas yield subtle variations in Sr-Nd-Os-Pb isotope space and have no clear variations with relative or absolute abundances of the HSE or trace elements. These results are consistent with a heterogeneous mantle plume model, with initial eruption of lavas from a primitive high-3He/4He mantle source ∼4 million years ago (Ma) to form Robinson Crusoe Island, which also led to 3He enrichment of the oceanic lithosphere, followed by eruption of Alexander Selkirk lavas from a more depleted mantle source at ∼2 Ma. Juan Fernandez lavas show that mantle heterogeneity preserved in OIB can occur over short-timescales (<2 Ma) and can impact lithospheric compositions, leading to eruption of rejuvenated lavas with unusual isotopic characteristics.

Yazdani, S, Castillo PR, Hassanzadeh J.  2018.  Crust-mantle interaction inferred from the petrology and Sr-Nd-Pb isotope geochemistry of Eocene arc lavas from the Kahrizak Mountains, north-central Iran. Lithos. 318:299-313.   10.1016/j.lithos.2018.08.018   AbstractWebsite

The Eocene volcanic rocks from the Kahrizak Mountains in north-central Iran are part of the Urumieh-Dokhtar magmatic arc, which runs parallel to the Main Zagros Thrust segment of the Neo-Tethys suture. These volcanic rocks, similar to those from eastern Pontides and northern Anatolia, Turkey, were mainly produced during the Eocene magmatic flare-up associated with the Arabia-Eurasia convergence. The rock suite includes basalt, trachyandesite/andesite and trachydacite/rhyolite lavas and pyroclastic deposits that evolved compositionally from calc-alkalic to shoshonitic. Their normalized trace element concentration patterns are moderately enriched in light rare earth element and depleted in high field-strength elements (HFSE; e.g., Nb, Ta, Ti). They have narrow ranges of initial Pb isotopic ratios and Nd-143/Nd-144(j), but highly variable Sr-87/Sr-86(j). The new analyses indicate that the parental magmas of the volcanic rocks were derived from a mantle source that had been enriched by fluids released from a subducted oceanic slab. The fluids introduced significant amounts of large ion lithophile elements, but negligible HFSE to the source. The parental magmas underwent fractional crystallization and assimilation of upper crustal materials to produce the range of volcanic rocks. Integration of new analyses with regional data suggests that the Eocene volcanic rocks from north-central Iran, together with-coeval volcanic rocks in eastern Pontides and northern Anatolia, were most probably derived from a lithospheric mantle source that had been previously metasomatized by fluids derived from a subducted slab before and during the Arabia-Eurasia collision. (C) 2018 Elsevier B.V. All rights reserved.

Castillo, PR, Macisaac C, Perry S, Veizer J.  2018.  Marine carbonates in the mantle source of oceanic basalts: Pb isotopic constraints. Scientific Reports. 8   10.1038/s41598-018-33178-4   AbstractWebsite

For almost fifty years, geochemists have been interpreting the clues from Pb isotopic ratios concerning mantle composition and evolution separately. The Pb isotopes of ocean island basalts (OIB) indicate that their mantle source is heterogeneous, most likely due to the presence of end-components derived from recycled crust and sediment. Some OIB have unusually high Pb-206/Pb-204 coming from one of the end-components with a long time-integrated high U-238/Pb-204 or mu (HIMU). Most OIB and many mid-ocean ridge basalts (MORB) also have high Pb-206/Pb-204, indicating a HIMU-like source. Moreover, measured Th-232/U-238 (kappa) for most MORB are lower than those deduced from their Pb-208/Pb-204 and Pb-206/Pb-204. Such high mu and low kappa features of oceanic basalts are inconsistent with the known geochemical behavior of U, Pb and Th and temporal evolution of the mantle; these have been respectively termed the 1st and 2nd Pb paradox. Here we show that subducted marine carbonates can be a source for HIMU and a solution to the Pb paradoxes. The results are consistent with the predictions of the marine carbonate recycling hypothesis that posits the Pb isotopes of oceanic basalts indicate a common origin and/or magma generation process.

Panter, KS, Castillo P, Krans S, Deering C, McIntosh W, Valley JW, Kitajima K, Kyle P, Hart S, Blusztajn J.  2018.  Melt origin across a rifted continental margin: A case for subduction-related metasomatic agents in the lithospheric source of alkaline basalt, NW Ross Sea, Antarctica. Journal of Petrology. 59:517-557.   10.1093/petrology/egy036   AbstractWebsite

Alkaline magmatism associated with the West Antarctic rift system in the NW Ross Sea (NWRS) includes a north-south chain of shield volcano complexes extending 260km along the coast of Northern Victoria Land (NVL), numerous small volcanic seamounts located on the continental shelf and hundreds more within an 35 000km 2 area of the oceanic Adare Basin. New 40 Ar/39 Ar age dating and geochemistry confirm that the seamounts are of Pliocene-Pleistocene age and petrogenetically akin to the mostly middle to late Miocene volcanism on the continent, as well as to a much broader region of diffuse alkaline volcanism that encompasses areas of West Antarctica, Zealandia and eastern Australia. All of these continental regions were contiguous prior to the late-stage breakup of Gondwana at 100 Ma, suggesting that the magmatism is interrelated, yet the mantle source and cause of melting remain controversial. The NWRS provides a rare opportunity to study cogenetic volcanism across the transition from continent to ocean and consequently offers a unique perspective from which to evaluate mantle processes and the roles of lithospheric and sublithospheric sources for mafic alkaline magmas. Mafic alkaline magmas with > 6wt % MgO (alkali basalt, basanite, hawaiite, and tephrite) erupted across the transition from continent to ocean in the NWRS show a remarkable systematic increase in silica-undersaturation, P2O5, Sr, Zr, Nb and light rare earth element (LREE) concentrations, as well as LREE/HREE (heavy REE) and Nb/Y ratios. Radiogenic isotopes also vary, with Nd and Pb isotopic compositions increasing and Sr isotopic compositions decreasing oceanward. These variations cannot be explained by shallow-level crustal contamination or by changes in the degree of mantle partial melting, but are considered to be a function of the thickness and age of the mantle lithosphere. We propose that the isotopic signature of the most silica-undersaturated and incompatible element enriched basalts best represent the composition of the sub-lithospheric magma source with low 87 Sr/86 Sr ( 0 7030) and d 18 Oolivine ( 5 0&), and high 143 Nd/144 Nd ( 0 5130) and 206 Pb/204 Pb ( 20). The isotopic ` endmember' signature of the sub-lithospheric source is derived from recycled subducted materials and was transferred to the lithospheric mantle by small-degree melts (carbonate-rich silicate liquids) to form amphibole-rich metasomes. Later melting of the metasomes produced silica-undersaturated liquids that reacted with the surrounding peridotite. This reaction occurred to a greater extent as the melt traversed through thicker and older lithosphere continentward. Ancient and/ or more recent ( 550-100 Ma) subduction along the Pan-Pacific margin of Gondwana supplied the recycled subduction-related material to the asthenosphere. Melting and carbonate metasomatism were triggered during major episodes of extension beginning in the Late Cretaceous, but alkaline magmatism was very limited in its extent. A significant delay of 30 to 20 Myr between extension and magmatism was probably controlled by conductive heating and the rate of thermal migration at the base of the lithosphere. Heating was facilitated by regional mantle upwelling, possibly driven by slab detachment and sinking into the lower mantle and/ or by edge-driven mantle flow established at the boundary between the thinned lithosphere of the West Antarctic rift and the thick East Antarctic craton.

Truong, TB, Castillo PR, Hilton DR, Day JMD.  2018.  The trace element and Sr-Nd-Pb isotope geochemistry of Juan Fernandez lavas reveal variable contributions from a high-He-3/He-4 mantle plume. Chemical Geology. 476:280-291.   10.1016/j.chemgeo.2017.11.024   AbstractWebsite

The Juan Fernandez Islands in the southeastern Pacific are an atypical linear volcanic chain that exhibits a considerable range in He-3/He-4 ratios (8 to 18 RA, where RA is the He-3/He-4 ratio of air), but limited ranges of Sr-87/Sr-86 and Nd-143/Nd-144. Here we report new trace element abundance data and Sr-Nd-Pb isotope data for mafic lavas previously analyzed for their He-3/He-4 and He contents from the two main islands of Robinson Crusoe and Alexander Selkirk. Lavas from these islands have been previously grouped based on geochemical and petrological classification into Group I and III basalts, and Group II basanites. In general, samples have overlapping Sr-Nd-Pb isotope compositions that suggest a common, albeit slightly heterogeneous mantle source. In detail, the Group I and III tholeiitic and alkalic basalts have nearly identical incompatible trace element patterns, whereas the Group II basanites show elevated incompatible trace element abundances. Major and incompatible trace element modeling indicates that Group III basalts (He-3/He-4 = 7.8-9.5 R-A) from younger Alexander Selkirk Island were produced by the highest degree of partial melting (> 10%) of a common mantle source, followed by Group I basalts (13.6-18.0 RA) and Group II basanites (11.2-12.5 RA) from older Robinson Crusoe Island. The Pb-206/Pb-204 of Group I basalts and Group II basanites are slightly more radiogenic and limited in range (19.163 to 19.292) compared with those of Group III (18.939 to 19.221). The Group I and II lavas from Robinson Crusoe are consistent with an origin from the so-called focus zone (FOZO) mantle component, whereas the Alexander Selkirk basalts additionally contain contributions from a less-enriched or relatively depleted mantle component. Juan Fernandez lavas reveal limited ranges of Sr-Nd-Pb isotopes but variable He-3/He-4 as their parental magmas originated mainly from the FOZO component with high He-3/He-4 (> 9 R-A) and variably polluted with a depleted component with lower He-3/He-4 (ca. 8 R-A). Contributions from high-He-3/He-4 mantle sources to ocean island basalts can therefore vary both spatially and temporally, over meter to kilometer lengths and hundred to millionyear time scales, and may not be strongly correlated to radiogenic lithophile isotope systematics.

Liu, XJ, Xiao WJ, Xu JF, Castillo PR, Shi Y.  2017.  Geochemical signature and rock associations of ocean ridge-subduction: Evidence from the Karamaili Paleo-Asian ophiolite in east Junggar, NW China. Gondwana Research. 48:34-49.   10.1016/   AbstractWebsite

Subduction of active spreading ridges most likely occurs throughout Earth's history. Interaction or collision between spreading center and trench, with the active spreading ridge downgoing and shallowly being buried in subduction zone, results in low-pressure but high-temperature near-trench magmatism in the forearc and accretionary prism setting. The Central Asian region, a complex orogenic belt created during the evolution and closure of the Paleo-Asian Ocean (PAO) at similar to 1000-300 Ma, provides an ideal place to study the subduction of PAO spreading ridges beneath ancient continental margins. It had been suggested that the low-pressure and high-temperature mafic and intermediate to felsic magmas from the Karamaili ophiolite (KO) in the NE corner of the Junggar basin (NW China) in Central Asia were likely produced by ridge subduction (Liu et al., 2007). In this paper, we combine our new geochemical data with previous results to show that the geochemical characteristics of the bulk of KO mafic rocks range from arc basalt-like to mid-ocean ridge basalt-like and ocean island basalt-like. Their trace element patterns range from depleted to enriched in highly incompatible elements, but depleted in Nb and Ta, indicating a subduction-influenced origin. The KO intermediate to felsic rocks are talc-alkaline and boninitic in composition and have trace element signatures similar to the associated mafic rocks. The low Nb/Ta ratios of some of the mafic rocks and boninitic character of some of the intermediate to felsic rocks reflect a highly depleted source, perhaps due to prior backarc magmatism. Major and trace element models indicate complex fractional crystallization histories of parental KO magmas to generate both the mafic and intermediate to felsic rocks, but in general, crystal fractionation occurred at 1000 to 1200 degrees C and moderate to low (0.5 kbar to 10 kbar) pressure or <23 km depth. We conclude that the KO was formed in a forearc region of a subduction system that experienced ridge subduction. (C) 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Liu, XJ, Liang QD, Li ZL, Castillo PR, Shi Y, Xu JF, Huang XL, Liao S, Huang WL, Wu WN.  2017.  Origin of Permian extremely high Ti/Y mafic lavas and dykes from Western Guangxi, SW China: Implications for the Emeishan mantle plume magmatism. Journal of Asian Earth Sciences. 141:97-111.   10.1016/j.jseaes.2016.09.005   AbstractWebsite

Late Permian mafic flows and dikes are prominent features in and around the Western Guangxi region in southern China. Based on petrographic, geochemical and Sr-Nd isotopic data, the western Guangxi mafic rocks are geochemically akin to the Emeishan large igneous province (ELIP) high-Ti basalts, except that they possess extremely elevated Ti/Y ratios (750-2000). The Dy/Yb and Ti/Y vs. Dy/Dy* covariations of the mafic rocks indicate a garnet-controlled magmatic differentiation of a mafic melt at relatively great depth. The limited epsilon(Nd)(t) range from +0.41 to +1.81 also suggests minimal crustal contamination of such a melt. Geochemical modeling using TiO2/Yb vs. Nb/Yb and Zr/Y vs. Nb/Y projections indicate that the parental melts of the western Guangxi mafic rocks formed at a low degree (<5%) of partial melting at or over 3.5 GPa, consistent with a deep mantle plume source under a thick continental lithosphere. Thus, the Guangxi extremely high Ti/Y mafic rocks most likely represent a part of outer zone of the ELIP plume magmatism. Results of this study reinforce the previously proposed temporal and spatial distribution of the ELIP. (C) 2016 Elsevier Ltd. All rights reserved.

Shi, Y, Pei XL, Castillo PR, Liu XJ, Ding HH, Guo ZC.  2017.  Petrogenesis of the similar to 500 Ma Fushui mafic intrusion and Early Paleozoic tectonic evolution of the Northern Qinling Belt, Central China. Journal of Asian Earth Sciences. 141:74-96.   10.1016/j.jseaes.2016.09.003   AbstractWebsite

The Fushui mafic intrusion in the Qinling orogenic belt (QOB) is composed of meta-gabbro, meta-gabbrodiorite, diorite, and syenite. Most of these rocks are metamorphosed under the upper greenschist facies to lower amphibolite facies metamorphism. Zircon separates from eight samples have LA-ICP-MS U-Pb ages of 497-501 Ma which are taken to be the emplacement age of magmas that formed the Fushui intrusion. Most of the zircon grains exhibit negative epsilon(HF) values, correspond to T-DMS model ages of late Paleoproterozoic-early Mesoproterozoic or Neoproterozoic and suggest that the mafic rocks were most probably derived from mafic melts produced by partial melting of a previously metasomatized litho spheric mantle. The intrusion is not extensively contaminated by crustal materials and most chemical compositions of rocks are not modified during the greenschist to amphibolite-facies metamorhism. Rocks from the intrusion have primitive mantle-normalized trace element patterns with significant enrichment in light-REE and large ion lithophile elements (LILE) and depletion in high field-strength elements (HFSE). On the basis of the trace element contents, the Fushui intrusion was derived from parental magmas generated by <10% partial melting of both phlogopite-lherzolite and garnet-lherzolite mantle sources. These sources are best interpreted to be in a subduction-related arc environment and have been modified by fluids released from a subducting slab. The formation of the Fushui intrusion was related to the subduction of the Paleotethyan Shangdan oceanic lithosphere at similar to 500 Ma. (C) 2016 Elsevier Ltd. All rights reserved.

Castillo, PR.  2016.  A proposed new approach and unified solution to old Pb paradoxes. Lithos. 252:32-40.   10.1016/j.lithos.2016.02.015   AbstractWebsite

One of the most remarkable features of many and, perhaps, all oceanic basalts is that their Pb isotopic ratios ((206)pb/(204)pb, Pb-207/Pb-204 and (208)pb/Pb-204) are too radiogenic to be coming from the undifferentiated mantle or bulk silicate Earth. This has created three major concerns in the behavior of U, Th and Pb in the Earth's mantle that have been termed the Pb paradoxes. These are the unexpectedly long time-integrated high U/Pb (1st paradox), long time-integrated low Th/U (2nd paradox) and constant Ce/Pb and Nb/U (3rd paradox) in the mantle sources of oceanic basalts. The origins of such unexpected ratios have been the object of intense studies that produced several highly significant, but generally individualized results during the last four decades. Detailed analysis of available data shows that the paradoxes are closely interrelated as they all pertain to the mantle and have many common characteristic features. Thus, the Pb paradoxes constitute a system of equations that must be solved all together as each solution must satisfy every equation in the system. For example, compositional data for the voluminous mid-ocean ridge basalts (MORE) show that the 1st and 2nd paradoxes exhibit a long time-integrated enrichment of U and the Th/U and Nb/Th ratios are also constant. A single solution to simultaneously explain the paradoxes in MORE is possible if recycled materials with variable enrichments in incompatible trace elements, particularly U and its daughter Pb* plus Nb, Ce, and Th are added to or mixed with the depleted upper mantle. Significantly, a similar binary mixing solution has been proposed for the Pb paradoxes in ocean island basalts. (c) 2016 Elsevier B.V. All rights reserved.

Liu, XJ, Xu JF, Xiao WJ, Castillo PR, Shi Y, Wang SQ, Huo QY, Feng ZH.  2015.  The boundary between the Central Asian Orogenic belt and Tethyan tectonic domain deduced from Pb isotopic data. Journal of Asian Earth Sciences. 113:7-15.   10.1016/j.jseaes.2015.04.039   AbstractWebsite

A detailed comparison of the tectonic features of Central Asian Orogenic belt (CAOB) and Tethyan Tectonic domain (TTD) is of great significance to our understanding of the origin of global orogenic systems. Currently, there are many uncertainties in the general framework to fully define the tectonic properties of the CAOB and TTD. The Pb isotope data from Paleo-Asian Ocean (PAO) ophiolites in the CAOB and Tethyan ophiolites in the TTD allow us to conduct a detailed comparative study between these two global orogenic systems. Results of the study show the presence of an isotopic boundary between the different mantle domains and tectonic properties of the CAOB and TTD, with the Xinjiang region in the former representing the transition between the two systems. The distinctive Pb-208/Pb-204 isotope compositions of the PAO and Tethyan mantles suggest the existence of a long time-integrated lower Th/U reservoir beneath the CAOB compared to that beneath the TTD throughout the Paleozoic. Results thus suggest the distinct Pb isotope compositions of the PAO and Tethyan mantles are intimately related to the different magma tectonic processes that formed the CAOB and TTD. Based on plate tectonic reconstruction, the Neoproterozoic to Paleozoic evolution of the accretionary margins of the CAOB mimics the modern circum-Pacific Ocean rim. In this scenario, the PAO had a low Th/U mantle isotopic signature and the subduction of PAO crust gave rise to the circum-Pacific type accretionary orogen. On the other hand, the Tethys oceans produced the high Th/U mantle isotopic signatures in an evolving collisional orogen. Significantly, the generally radiogenic and juvenile Hf isotopic signature of the CAOB is consistent with an accretionary orogenic setting for PAO whereas the relatively more unradiogenic Hf isotopic signature of TTD is consistent with a collisional orogenic setting for Tethys oceans. Thus, our study sheds some light on the PAO evolution as well as the plate tectonic reconstruction of Central Asian and Tethyan orogens. (c) 2015 Elsevier Ltd. All rights reserved.

Yan, QS, Castillo P, Shi XF, Wang LL, Liao L, Ren JB.  2015.  Geochemistry and petrogenesis of volcanic rocks from Daimao Seamount (South China Sea) and their tectonic implications. Lithos. 218:117-126.   10.1016/j.lithos.2014.12.023   AbstractWebsite

The South China Sea (SCS) experienced three episodes of seafloor spreading and left three fossil spreading centers presently located at 18 degrees N, 17 degrees N and 15.5 degrees N. Spreading ceased at these three locations during magnetic anomaly 10, 8, and 5c, respectively. Daimao Seamount (16.6 Ma) was formed 10 my after the cessation of the 17 degrees N spreading center. Volcaniclastic rocks and shallow-water carbonate facies near the summit of Daimao Seamount provide key information on the seamount's geologic history. New major and trace element and Sr-Nd-Pb isotopic compositions of basaltic breccia clasts in the volcaniclastics suggest that Daimao and other SCS seamounts have typical ocean island basalt-like composition and possess a 'Dupal' isotopic signature. Our new analyses, combined with available data, indicate that the basaltic foundation of Daimao Seamount was formed through subaqueous explosive volcanic eruptions at 16.6 Ma. The seamount subsided rapidly (>0.12 mm/y) at first, allowing the deposition of shallow-water, coral-bearing carbonates around its summit and, then, at a slower rate (<0.12 mm/y). We propose that the parental magmas of SCS seamount lavas originated from the Hainan mantle plume. In contrast, lavas from contemporaneous seamounts in other marginal basins in the western Pacific are subduction-related. (C) 2015 Elsevier B.V. All rights reserved.

Castillo, PR.  2015.  The recycling of marine carbonates and sources of HIMU and FOZO ocean island basalts. Lithos. 216:254-263.   10.1016/j.lithos.2014.12.005   AbstractWebsite

Many, and perhaps all, oceanic island basalts (OIB) clearly contain a component of crustal materials that have been returned to the mantle through subduction or other processes. One of the first recycled materials to be identified as a potential source of OIB was mid-ocean ridge basalt (MORE), and this was later fine-tuned as having a long time-integrated (b.y.) high U/Pb ratio or high mu (HIMU) and producing OIB with the most radiogenic Pb isotopic ratios (Pb-206/Pb-204 > 20). However, it is becoming more evident that the compositional connection between subducted MORE and HIMU basalts is problematic. As an alternative hypothesis, a small amount (a few %) of recycled Archaean marine carbonates (primarily CaCO3) is proposed to be the main source of the distinct Pb-206/Pb-204, Pb-207/Pb-204 and Sr-87/Sr-86 isotopic and major-trace element compositions of classic HIMU and post-Archaean marine carbonates for younger HIMU or the so-called FOZO mantle source. As an extension of the hypothesis, a conceptual model that combines the separate evolutionary histories of ancient oceanic lithosphere, which is the source of OIB, and upper mantle, which is the source of MORE, is also proposed. The model claims that FOZO mainly consists of the lithospheric mantle portion of the ancient metamorphosed oceanic slabs that have accumulated in the deep mantle. Such an ultramafic source is geochemically depleted due to prior extraction of basaltic melt plus removal of the enriched subduction component from the slab through dehydration and metamorphic processes. Combined with other proposed models in the literature, the conceptual model can provide reasonable solutions for the Pb-208/Pb-204, Nd-143/Nd-144, Hf-176/Hf-177 and He-3/He-4 isotopic paradoxes or complexities of oceanic lavas. Although these simultaneous solutions for individual paradoxes are qualitative and non-unique, these are unified under a single, marine carbonate recycling hypothesis. 10 2014 Elsevier B.V. All rights reserved.

Castillo, PR, Hilton DR, Halldórsson SA.  2014.  Trace element and Sr-Nd-Pb isotope geochemistry of Rungwe Volcanic Province, Tanzania: Implications for a superplume source for East Africa Rift magmatism. Frontiers in Earth Science. 2   10.3389/feart.2014.00021   AbstractWebsite

The recently-discovered high, plume-like 3He/4He ratios at Rungwe Volcanic Province (RVP) in southern Tanzania, similar to those at the Main Ethiopian Rift in Ethiopia, strongly suggest that magmatism associated with continental rifting along the entire East African Rift System (EARS) has a deep mantle contribution (Hilton et al., 2011). New trace element and Sr-Nd-Pb isotopic data for high 3He/4He lavas and tephras from RVP can be explained by binary mixing relationships involving Early Proterozoic (±Archaean) lithospheric mantle, present beneath the southern EARS, and a volatile-rich carbonatitic plume with a limited range of compositions and best represented by recent Nyiragongo lavas from the Virunga Volcanic Province also in the Western Rift. Other lavas from the Western Rift and from the southern Kenyan Rift can also be explained through mixing between the same endmember components. In contrast, lavas from the northern Kenyan and Main Ethiopian rifts can be explained through variable mixing between the same mantle plume material and Middle to Late Proterozoic lithospheric mantle, present beneath the northern EARS. Thus, we propose that the bulk of EARS magmatism is sourced from mixing among three endmember sources: Early Proterozoic (±Archaean) lithospheric mantle, Middle to Late Proterozoic lithospheric mantle and a volatile-rich carbonatitic plume with a limited range of compositions. We propose further that the African Superplume, a large, seismically anomalous feature originating in the lower mantle beneath southern Africa, influences magmatism throughout eastern Africa with magmatism at RVP and the Main Ethiopian Rift representing two different heads of a single mantle plume source. This is consistent with a single mantle plume origin of the coupled He-Ne isotopic signatures of mantle-derived xenoliths and/or lavas from all segments of the EARS (Halldórsson et al., 2014).

Yan, QS, Shi XF, Castillo PR.  2014.  The late Mesozoic-Cenozoic tectonic evolution of the South China Sea: A petrologic perspective. Journal of Asian Earth Sciences. 85:178-201.   10.1016/j.jseaes.2014.02.005   AbstractWebsite

This paper presents a review of available petrological, geochonological and geochemical data for late Mesozoic to Recent igneous rocks in the South China Sea (SCS) and adjacent regions and a discussion of their petrogeneses and tectonic implications. The integration of these data with available geophysical and other geologic information led to the following tectono-magmatic model for the evolution of the SCS region. The geochemical characteristics of late Mesozoic granitic rocks in the Pearl River Mouth Basin (PRMB), micro-blocks in the SCS, the offshore continental shelf and Dalat zone in southern Vietnam, and the Schwaner Mountains in West Kalimantan, Borneo indicate that these are mainly I-type granites plus a small amount of S-type granites in the PRMB. These granitoids were formed in a continental arc tectonic setting, consistent with the ideas proposed by Holloway (1982) and Taylor and Hayes (1980, 1983), that there existed an Andean-type volcanic arc during later Mesozoic era in the SCS region. The geochonological and geochemical characteristics of the volcanics indicate an early period of bimodal volcanism (60-43 Ma or 32 Ma) at the northern margin of the SCS, followed by a period of relatively passive style volcanism during Cenozoic seafloor spreading (37 or 30-16 Ma) within the SCS, and post-spreading volcanism (tholeiitic series at 17-8 Ma, followed by alkali series from 8 Ma to present) in the entire SCS region. The geodynamic setting of the earlier volcanics was an extensional regime, which resulted from the collision between India and Eurasian plates since the earliest Cenozoic, and that of the post-spreading volcanics may be related to mantle plume magmatism in Hainan Island. In addition, the nascent Hainan plume may have played a significant role in the extension along the northern margin and seafloor spreading in the SCS. (C) 2014 Elsevier Ltd. All rights reserved.

Liu, XJ, Xu JF, Castillo PR, Xiao WJ, Shi Y, Feng ZH, Guo L.  2014.  The Dupal isotopic anomaly in the southern Paleo-Asian Ocean: Nd-Pb isotope evidence from ophiolites in Northwest China. Lithos. 189:185-200.   10.1016/j.lithos.2013.08.020   AbstractWebsite

It has been suggested that the Dupal isotopic anomaly in the mantle can be traced in the Paleozoic ophiolites from the Neo- and Paleo-Tethyan Ocean (275-350 Ma). The Karamaili ophiolite (KO) and Dalabute ophiolite (DO) in the eastern and western corners, respectively, of the Junggar basin in NW China represent remnants of the relatively older (>350 Ma) Paleo-Asian Ocean (PAO) crust. Thus, these ophiolites can provide additional constraints on the long-term composition and evolution of the Paleozoic suboceanic mantle. We present new major-trace element and Sr, Nd and high-precision Pb isotope data for the basalts, gabbros and a plagioclase separate from the KO and DO. Our results indicate that the PAO crust indeed has a Dupal-like isotopic signature. In detail, all samples have relatively low epsilon(Nd(t)) and high Pb-208/Pb-204((t)) for given Pb-206/(204) Pb-(t) ratios (i.e., positive Delta 8/4 values), similar to the Dupal isotopic characteristics of Indian Ocean mid-ocean ridge basalts (MORB). The trace element signature of DO mafic rocks is similar to that of normal- and enriched-MORB whereas that of the KO is transitional between MORB and arc basalt. Therefore, the DO mantle domain reflects the PAO asthenosphere and the KO domain additionally shows the influence of the subduction process. Geochemical modeling using Th/Nd as well as Nd and Pb isotopic ratios indicates that up to 2% subduction component had been added to a depleted Indian MORB-type mantle to produce the bulk of KO rocks. The subduction component in the KO rocks consisted of variable proportions of <= 1% partial melt of unradiogenic sediment similar to modern lzu-Bonin trench sediment and hydrous fluid dehydrated from the subducted altered oceanic crust. The Devonian asthenospheric mantle beneath the southern PAO is isotopically heterogeneous, but lends support to the idea that the Dupal isotopic anomaly existed prior to the opening of the Indian Ocean. Finally, plate tectonic reconstruction indicates that the anomaly was present in the Neo- and Paleo-Tethyan oceans in the southern hemisphere and in the southern part of PAO in the northern hemisphere during the late Paleozoic. C) 2013 Elsevier B.V. All rights reserved.

Hahm, D, Hilton DR, Castillo PR, Hawkins JW, Hanan BB, Hauri EH.  2012.  An overview of the volatile systematics of the Lau Basin - Resolving the effects of source variation, magmatic degassing and crustal contamination. Geochimica Et Cosmochimica Acta. 85:88-113.   10.1016/j.gca.2012.02.007   AbstractWebsite

The Lau Basin erupts lavas with a range of geochemical features reflecting a complex history of interaction involving different mantle sources. The Valu Fa Ridge (VFR) and Mangatolu Triple Junction (MTJ) region have lavas with arc-like characteristics, Niuafo'ou Island (NV), Peggy Ridge and Central and Eastern Lau Spreading Centers (PR, CLSC and ELSC) erupt mid-ocean ridge basalt (MORB)-like volcanics, whereas the Rochambeau Bank (RB) has features akin to ocean island basalt (OIB). To characterize the volatile systematics of these various regions, we report a comprehensive study of 39 submarine lavas from these various eruptive centers encompassing analyses of the noble gases (He, Ne, and Ar) and carbon (CO2) - both isotopes and abundances - together with other major volatile phases (H2O, S, Cl, and F). Helium isotope ratios of the NV, MTJ, CLSC, and ELSC are MORB-like for the most part except for differentiated lavas that tend to have lower, more radiogenic He-3/He-4 values. The RB has considerably higher He-3/He-4 ratios (up to 23 R-A in this work) which extend as far south as the PR. The influence of 'plume-like' sources in the RB is also apparent in Ne isotopes: RB samples follow a trend similar to Hawaiian basalts in 3-isotope neon space. However, RB lavas have lower Ar-40/Ar-36 (300-730) and higher [Ar-36] than CLSC and ELSC, suggesting greater air contamination. Elemental He/Ne ratios (He-3/Ne-22(S) and He-4/Ne-21* where S = solar and * = nucleogenic) are high throughout the Lau Basin and identify the Lau mantle as one of only two high He-3/He-4 provinces worldwide with such an enrichment of He relative to Ne. Magmatic CO2 and d C-13 fall in the range 7-350 ppm and -28 parts per thousand to -6 parts per thousand, respectively. RB lavas have less [CO2] and slightly lower d C-13 than CLSC and ELSC. The lowest values are found among MTJ lavas. These lavas also have the highest [H2O], [F], [Cl], and [S] whereas the PR, ELSC and CLSC have the lowest. RB has intermediate [H2O]. We estimate primary [CO2] in primary melts using [CO2]-delta C-13 relationships, and find that RB lavas have higher [CO2] (similar to 935 +/- 168 ppm) than ELSC/CLSC (638 +/- 115 ppm). They also possess higher initial delta C-13 values, consistent with observations at other hotspot-related localities. However, there are no discernible differences in primary CO2/Nb ratios between mantle sources characterized by high He-3/He-4 and MORB-like ratios. On the other hand, reconstructed values are considerably higher than that envisaged for depleted MORB mantle based on olivine-hosted melt inclusions. (C) 2012 Elsevier Ltd. All rights reserved.

Castillo, PR.  2012.  Adakite petrogenesis. Lithos. 134:304-316.   10.1016/j.lithos.2011.09.013   AbstractWebsite

Adakite was originally proposed as a genetic term to define intermediate to high-silica, high Sr/Y and La/Yb volcanic and plutonic rocks derived from melting of the basaltic portion of oceanic crust subducted beneath volcanic arcs. It was also initially believed that adakite only occurs in convergent margins where young and, thus, still hot oceanic slabs are being subducted. Currently, adakite covers a range of arc rocks ranging from primary slab melt, to slab melt hybridized by peridotite, to melt derived from peridotite metasomatized by slab melt. Adakites can occur in arc settings where unusual tectonic conditions can lower the solidi of even older slabs and their source also includes subducted sediments. Results of adakite studies have generated controversies due to (1) the specific genetic definition of adakite but its reliance on trace element chemistry for its distinguishing characteristics, (2) curious association of adakite with alkalic rocks enriched in high field-strength elements and Cu-Au mineral deposits and (3) existence of adakitic rocks produced through other petrogenetic processes. Other studies have shown that adakitic rocks and a number of the previously reported adakites are produced through melting of the lower crust or ponded basaltic magma, high pressure crystal fractionation of basaltic magma and low pressure crystal fractionation of water-rich basaltic magma plus magma mixing processes in both arc or non-arc tectonic environments. Thus, although adakite investigations enrich our understanding of material recycling and magmatic processes along convergent margins, economic deposits and crustal evolutionary processes, the term adakite should be used with extreme caution. (C) 2011 Elsevier B.V. All rights reserved.

Yan, QS, Castillo PR, Shi XF.  2012.  Geochemistry of basaltic lavas from the southern Lau Basin: input of compositionally variable subduction components. International Geology Review. 54:1456-1474.   10.1080/00206814.2012.664031   AbstractWebsite

We present new major element, trace element, and Sr-Nd-Pb isotope data for 18 basaltic lavas and six glasses collected in situ from the Eastern Lau Spreading Centre (ELSC) and the Valu Fa Ridge (VFR). All lava samples are aphanitic and contain rare plagioclase and clinopyroxene microlites and microphenocrysts. The rocks are sub-alkaline and range from basalt and basaltic andesite to more differentiated andesite. In terms of trace element compositions, the samples are transitional between typical normal mid-ocean ridge basalt (MORB) and island arc basalt. Samples from the VFR have higher large ion lithophile element/high field strength element ratios (e. g. Ba/Nb) than the ELSC samples. VFR and ELSC Sr-Nd isotopic compositions plot between Indian MORB and Tonga arc lavas, but VFR samples have higher Sr-87/Sr-86 for a given Nd-143/Nd-144 ratio than ELSC analogues. The Pb isotopic composition of ELSC lavas is more Indian MORB-like, whereas that of VFR lavas is more Pacific MORB-like. Our new data, combined with literature data for the Central Lau Spreading Centre, indicate that the mantle beneath the ELSC and VFR spreading centres was originally of Pacific type in composition, but was displaced by Indian-type mantle as rifting propagated to the south. The mantle beneath the spreading centres also was variably affected by subduction-induced metasomatism, mainly by fluids released from the altered, subducting oceanic crust; the influence of these components is best seen in VFR lavas. To a first approximation, the effects of underflow on the composition and degree of partial melting of the mantle source of Lau spreading centre lavas inversely correlate with distance of the spreading centres from the Tonga arc. Superimposed on this general process, however, are the effects of the local geographic contrasts in the composition of subduction components. The latter have been transferred mainly by dehydration-generated fluids into the mantle beneath the Tonga supra-subduction zone.

Tian, LY, Castillo PR, Hilton DR, Hawkins JW, Hanan BB, Pietruszka AJ.  2011.  Major and trace element and Sr-Nd isotope signatures of the northern Lau Basin lavas: Implications for the composition and dynamics of the back-arc basin mantle. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008791   AbstractWebsite

We present new major element, trace element and Sr-Nd isotope analyses of volcanic glasses from Mangatolu Triple Junction (MTJ), Peggy Ridge (PR), Rochambeau Bank (RB), and Niuafo'ou Island (NF) within the northern Lau Basin (NLB). Lavas from MTJ range from tholeiitic basalts to basaltic andesites and andesites: such a lava series can be ascribed to fractional crystallization. Lavas from NF, RB and PR are mainly tholeiitic basalts save for two transitional basalts from RB. The lavas came from a compositionally heterogeneous mantle that exhibits compositional features similar to those of the mantle source of Indian mid-ocean basalt and bears the influence of both subduction and ocean island basalt (OIB) components. The subduction components consist mainly of fluid dehydrated from subducted oceanic crust and a minor amount of sediment melt. The geochemically enriched signature of enriched RB, PR and NF lavas comes from two OIB end-member components, most likely derived from enriched Samoan mantle plume materials leaking into NLB. The Rochambeau Rifts (RR)-RB corridor receives the greatest, although still variable, influence from the mantle source of Samoan shield magmatism whereas the outlying PR and NF regions experience a Samoan plume post-erosional type of magmatism. The relatively recent mixing of Samoan plume materials with the subduction-metasomatized Indian-type mantle may be responsible for some of the observed complex relationships between noble gases and other geochemical tracers in some NLB lavas.

Tian, LY, Castillo PR, Lonsdale PF, Hahm D, Hilton DR.  2011.  Petrology and Sr-Nd-Pb-He isotope geochemistry of postspreading lavas on fossil spreading axes off Baja California Sur, Mexico. Geochemistry Geophysics Geosystems. 12   10.1029/2010gc003319   AbstractWebsite

Postspreading volcanism has built large seamounts and volcanic ridges along the short axes of a highly segmented part of the East Pacific Rise crest that ceased spreading at the end of the middle Miocene, offshore Baja California Sur, Mexico. Lava samples from Rosa Seamount, the largest volcano, are predominantly alkalic basalts, mugearites, and benmoreites. This lavas series was generated through fractional crystallization and is compositionally similar to the moderately alkalic lava series in many oceanic islands. Samples from volcanic ridges at three adjacent failed spreading axes include mildly alkalic, transitional, and tholeiitic basalts and differentiated trachyandesites and andesite. The subtle but distinct petrologic and isotopic differences among the four sites may be due to differences in the degree of partial melting of a common, heterogeneous source. Postspreading lavas from these four abandoned axes off Baja California Sur together with those from other fossil spreading axes and from seamount volcanoes that grew on the East Pacific Rise flanks define a compositional continuum ranging from normal mid-ocean ridge basalt (NMORB)-like to ocean island basalt (OIB)-like. We propose that the compositional spectrum of these intraplate volcanic lavas is due to different degrees of partial melting of the compositionally heterogeneous suboceanic mantle in the eastern Pacific. A large degree of partial melting of this heterogeneous mantle during vigorous mantle upwelling at an active spreading center produces NMORB melts, whereas a lesser degree of partial melting during weak mantle upwelling following cessation of spreading produces OIB-like melts. The latter melts have a low (<8 R(A)) (3)He/(4)He signature indicating their formation is different from that of OIBs from major "hot spot" volcanoes in the Pacific with high (3)He/(4)He ratios, such as Hawaii and Galapagos.

Castillo, PR, Clague DA, Davis AS, Lonsdale PF.  2010.  Petrogenesis of Davidson Seamount lavas and its implications for fossil spreading center and intraplate magmatism in the eastern Pacific. Geochemistry Geophysics Geosystems. 11   10.1029/2009gc002992   AbstractWebsite

Seafloor spreading causes abundant magmatism along active ocean spreading centers, but the cause of magmatism along fossil spreading centers is enigmatic. Samples collected from Davidson Seamount, a typical volcanic ridge along an abandoned spreading center in the eastern Pacific, consist of an alkalic basalt to trachyte lava series; transitional basalts were sampled from another part of the abandoned axis, 20 km from the seamount. All samples experienced complex fractional crystallization prior to eruption, but they all share a common, compositionally heterogeneous mantle source. The parental magmas of the transitional basalts were produced from this source at higher degree of melting than those of the alkalic lava series. The composition of Davidson lavas overlaps with those of ridges along other fossil spreading centers and isolated near-and off-ridge seamounts in the eastern Pacific. Together they define a compositional continuum ranging from tholeiitic, normal mid-ocean ridge basalt (MORB)-like to alkalic, ocean island basalt (OIB)-like, similar to lavas that form linear island chains and ridges. We propose that this entire compositional spectrum of intraplate lavas that do not form linear volcanic chains in the eastern Pacific results from variations in the degree of partial melting of a common, compositionally heterogeneous mantle source. This source consists of more easily melted, geochemically enriched components of varying sizes and amounts embedded in a depleted lherzolitic matrix. Large degree of partial melting produces normal MORB-like melts represented by some near-ridge seamount lavas, whereas small degree of melting produces OIB-like fossil spreading center lavas. The small degree of partial melting beneath recently abandoned spreading centers results from either buoyancy-driven decompression melting of the hot lithospheric and asthenospheric mantle material beneath active spreading centers or rapid motion, with respect to the underlying asthenosphere, of abandoned spreading axes that are thickening over a fertile mantle. Mid-Tertiary volcanic rocks in coastal California, which are compositionally akin to intraplate lavas, are interpreted to be small degree partial melts of the same compositionally heterogeneous sub-Pacific mantle that has upwelled through windows in a subducted slab.

Macpherson, CG, Chiang KK, Hall R, Nowell GM, Castillo PR, Thirlwall MF.  2010.  Plio-Pleistocene intra-plate magmatism from the southern Sulu Arc, Semporna peninsula, Sabah, Borneo: Implications for high-Nb basalt in subduction zones. Journal of Volcanology and Geothermal Research. 190:25-38.   10.1016/j.jvolgeores.2009.11.004   AbstractWebsite

New analyses of major and trace element concentrations and Sr, Nd and Pb isotopic ratios are presented for Plio-Pleistocene basalts and basaltic andesites from the Semporna peninsula in Sabah, Borneo, at the southern end of the Sulu Arc. Depletion of high field strength elements (HFSE), which is characteristic of many subduction-related magmatic suites, is present in more evolved Semporna rocks but is associated with radiogenic Sr and Pb, and less radiogenic Nd isotopic ratios and results from contamination of mafic melt by, possibly ancient, crustal basement. The most mafic lavas from Semporna, and elsewhere in the Sulu Arc, display no HFSE depletion relative to other elements with similar compatibility. High-Nb basalt from Semporna formed when mantle resembling the source of Ocean Island Basalt (OIB) upwelled into lithospheric thin spots created during earlier subduction. This mantle did not experience enrichment by fluids or melt derived from subducted crust. The presence of similar lavas throughout the Sulu Arc and around the South China Sea suggests that the OIB-like component resides in the convecting upper mantle. Depletion of light rare earth elements, with respect to other incompatible elements, throughout the Sulu Arc could result from melt-mantle interaction during magma transport through the lithosphere. Such depletion is absent in suites from the South China Sea, where magma probably migrated along large, lithosphere-penetrating structures. Semporna high-Nb basalts are not associated with adakitic magmatism which is a frequent, but not ubiquitous, association in some active subduction zones. Both geochemical signatures are developed early in the history of a melt pulse, either in the source (high-Nb basalt) or during deep differentiation (adakite). Preservation of these distinctive geochemical signatures is favoured in settings that minimise (1) interaction with other, more copious melt types, and/or (2) subsequent differentiation in the shallow crust. Where found, the high-Nb basalt-adakite association is a result of transport through favourable lithospheric conditions and not due to any link between their mantle Sources. (C) 2009 Elsevier B.V. All rights reserved.

Castillo, PR, Lonsdale PF, Moran CL, Hawkins JW.  2009.  Geochemistry of mid-Cretaceous Pacific crust being subducted along the Tonga-Kermadec Trench: Implications for the generation of arc lavas. Lithos. 112:87-102.   10.1016/j.lithos.2009.03.041   AbstractWebsite

The Pacific Plate is currently being subducted at a rapid rate beneath the Indo-Australian Plate along the Tonga-Kermadec Trench in the southwestern Pacific. It has long been assumed that the lithosphere being subducted is relatively old and homogeneous in composition. Basaltic lavas dredged from the upper crust of the incoming lithosphere along the length of the trench are mid- to late-Cretaceous in age. Although the samples are mainly N-MORB, they range from tholeiitic to alkalic basalts. Concentrations of incompatible trace elements show a high degree of variability (e.g., Ba = 8 to 270 ppm, Rb = 0.4 to 39 ppm, Sr = 60 to 625 ppm, and Nd = 2.3 to 25 ppm). Neodymium, Sr and Pb isotopic data also show wide ranges (epsilon(Nd)(T) = 5.7-11.2; (87)Sr/(86)Sr(i) = 0.70224-0.70311; (206)Pb/(204)Pb(i) = 17.84-20.22). More importantly, the basaltic crust being subducted displays a latitudinal compositional variation that is similar to that shown by the Tonga-Kermadec arc lavas. Previous studies have proposed that the variably depleted sub-arc mantle, which was preconditioned through a backarc melt extraction or displacement process, is mainly responsible for the latitudinal variation in the Tonga-Kermadec arc lavas. However, our new results suggest a greater role of the lithospheric input into the source of arc lavas. The three end-member possibilities linking the latitudinal variation of the lithospheric input to the source of arc lava output are: (I) the mantle wedge beneath the volcanic arc on the west side of the trench, the main source of Tonga-Kermadec arc lavas, is a western extension of the Cretaceous Pacific upper mantle east of the trench; (2) the altered oceanic crust melts and the resultant slab melt modifies the mantle source of arc lavas; and (3) fluids dehydrated from the altered oceanic crust effectively transfer the compositional signature of the subducted slab into the mantle source of arc lavas. (C) 2009 Elsevier B.V. All rights reserved.

Regelous, M, Niu YL, Abouchami W, Castillo PR.  2009.  Shallow origin for South Atlantic Dupal Anomaly from lower continental crust: Geochemical evidence from the Mid-Atlantic Ridge at 26 degrees S. Lithos. 112:57-72.   10.1016/j.lithos.2008.10.012   AbstractWebsite

We measured trace element concentrations and Pb isotope compositions of fresh volcanic glass samples from the Mid-Atlantic Ridge at 26 degrees S, and from nearby off-axis seamounts. The samples have previously been studied for major elements and Sr-Nd-He isotopes. All samples are depleted MORB, and include some of the most incompatible trace element depleted lavas yet reported from the Atlantic. The seamount lavas are more depleted in highly incompatible elements than the axial lavas, but have high Sr, Pb and Eu concentrations, relative to REE of similar incompatibility. The lavas with the highest Sr/Nd, Pb/Ce and Eu/Eu* have the highest (3)He/(4)He (up to 11.0 R/RA) ratios and the lowest incompatible trace element concentrations. They also have the highest (87)Sr/(86)Sr (up to 0.7036) and (208)Pb/(204)Pb for a given (206)Pb/(204)Pb ratio, which are characteristics of lavas from the Dupal Anomaly in the South Atlantic, and of many EM-1 type intraplate lavas generally. Our data place constraints on the origin of the Dupal Anomaly. The enrichments in Sr, Pb and Eu, together with the low Ca/Al ratios of the seamount lavas indicate that their mantle source consists of material that at one time contained plagioclase, and must therefore have resided at crustal pressures. We argue that the trace element and isotopic compositions of the seamount lavas are best explained by derivation from a mantle source contaminated with lower continental crust, which was introduced into the upper mantle during continental rifting and breakup in the South Atlantic. Our results support previous suggestions that the Dupal Anomaly in the South Atlantic has a relatively recent, shallow origin in lower continental crust and continental lithospheric mantle, rather than in recycled material supplied from the deeper mantle by plumes. Plate reconstructions place the Parana-Etendeka flood basalt province over the central part of the Dupal Anomaly at the time of rifting of South America and Africa at 134 Ma. The flood basalts which have undergone the least crustal-level contamination also have extreme Dupal compositions. We speculate that delamination of dense lower continental crust during continental rifting causes flood basalt magmatism, whilst variably polluting the upper oceanic mantle with continental material. (C) 2008 Elsevier B.V. All rights reserved.